1. Technical Field
The present disclosure relates generally to offline LED-driving circuitry and, more specifically, to an apparatus and method for compensating for input voltage variation and increasing power factor of single-stage embodiments of offline LED drivers.
2. Introduction
With the rapid development of high brightness Light Emitting Diodes (LEDs), the application of Solid State Lighting (SSL) begins to broaden in scope, particularly in regards to residential markets. For example, there is a relatively large potential market for residential application of SSL in a Compact Fluorescent Lamp (CFL) retrofit embodiment. Accordingly, the standardization of SSL products encourages growth of the market. In September 2007, the US Department of Energy (DOE) issued its Energy Star® specifications for SSL products, requiring the power factor of the power supply to be higher than 0.7 for residential application.
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load to the apparent power, and is represented as a number between 0 and 1 (sometimes expressed as a percentage, e.g. 0.75 pf=75% pf). Real power is the capacity of the circuit for performing work in a particular time, while apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power may be greater than the real power.
Cost, size, and reliability are significant factors impacting CFL retrofit applications. To achieve a high power factor, a passive or active Power Factor Corrector (PFC) may be used. Generally, PFCs control the amount of power drawn by a load in order to obtain the greatest power factor possible. Passive PFCs typically require large passive components which inhibit use within the small environment required for a retrofit application. The traditional active PFC circuit controls the input current of the load such that the current waveform is proportional to the mains voltage waveform. However, active PFCs typically require a two-stage topology (e.g. boost stage for PCF, then buck or flyback for the current regulation of the LEDs), wherein the cost of a two-stage application is substantially greater than the cost of a single-stage application.
In a document entitled “A Single-Stage Power Converter for a Large Screen LCD Back-Lighting,” published at the 2006 Applied Power Electronics Conference and Exposition and incorporated herein by reference, In-Hwan Oh presents a single-stage converter for LCD back-lighting using LEDs. The concept may be applied to CFL retrofit applications; however, the method disclosed to improve power factor causes significant power variation when the input voltage varies. Oh relies on a delay caused by an RC filter used for current sensing to shape the current, which is impacted by the amplitude of the input voltage.
3. Description of Related Art
Reference is made to FIG. 1 which shows a circuit diagram for a prior art power supply for an LED light source providing feedback control with isolation. The power supply, further disclosed in U.S. Pat. No. 6,577,512 to Tripathi and incorporated herein by reference, uses a flyback transformer 124 with current feedback through a power factor corrector 128 to supply power to a variable number of LED light sources 126. The flyback converter controls the current to the LEDs 126 at a desired value, while the PFC 128 supplies a gate drive signal to MOSFET Q1. MOSFET Q1 supplies a transformer control signal, adjusting the current flow through winding W1 of transformer 124 to match the LEDs 126 current demand until the sensed current signal and reference current signal are equal at current controller 130, such that the feedback error signal goes to zero. Although the flyback transformer with power factor corrector configuration has been widely used to provide isolated fixed voltage DC power sources with high line power factors, it requires LED current sensing and feedback, and thus, manufacture of the circuit is complex and costly.
Not every application requires isolation, however. A simple, non-isolated flyback configuration is provided in FIG. 2, and is further disclosed in U.S. Pat. No. 6,304,464 to Jacobs, incorporated herein by reference. The schematic illustrated in FIG. 2 relates to a circuit arrangement for operating a semiconductor light source, wherein the converter is a flyback converter with a switching element T1 connected in series with a transformer L2 provided with a primary winding L21 and a secondary winding L22. The application illustrated in FIG. 2 requires a current-measuring impedance R4 to sense the LED current, and diodes Z1/D1 to clamp the leakage energy, thereby reducing the efficiency of the circuit. As such, there is a need to provide simple, cost-effective circuitry to drive LED sources operating in constant power mode to eliminate LED current sensing, thereby ensuring a more efficient, high power factor circuit.